High energy (MeV) X-ray radiation is used to scan cargo containers and air shipments for contraband, verification of manifests for customs, and duty collection. The items of concern may depend on the individual customs agencies for a country, or individual ports. X-ray radiation may be used to non-intrusively examine cargo containers and other objects for illegal drugs, weapons, explosives, chemical agents, and/or biological agents. Radioactive, fissionable, fissile, and fertile materials, including Special Nuclear Material (“SNM”) that may be used to manufacture atomic devices, including dirty bombs, may also be smuggled in such objects.
Fissile materials, such as uranium-235, uranium-233, and plutonium-239, may undergo fission by the capture of a photon or slow (thermal) neutron of sufficient energy. Photon-induced fission is referred to as photofission. Fission may also be induced by lower energy photons and neutrons by barrier penetration at a lower rate than photofission.
Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. For example, uranium-238 may be converted into plutonium-239, and thorium-232 may be converted into uranium-233. Fissionable, fissile, and fertile materials are referred to herein as “nuclear material.”
Special Nuclear Material (“SNMs”), which more readily undergo fission than other fissile materials, are defined by the U.S. Nuclear Regulatory Commission to include plutonium, uranium-233, and uranium enriched in the isotopes of uranium-233 or -235. Radioactive materials, certain of which may have lower atomic numbers than nuclear materials (cobalt-60, for example, has an atomic number of 27), are typically shielded by high atomic number materials, such as lead (Z=82), tungsten (Z=74), and molybdenum (Z=42).
SNM may undergo fission by absorbing a photon having energy above a fission threshold of the particular SNM. SNM have fission thresholds of about 5.8 to 6.0 MeV. Photofission releases about 200 MeV of energy in the form of high energy neutrons, gamma rays, excited fission fragments, and kinetic energy transferred to fission fragments. The high energy neutrons and gamma rays are referred to herein as “prompt neutrons” and “prompt gamma rays,” respectively, because they are released very soon (on the order of 10−15 to 10−12 seconds) after fission. On average, 6 to 7 prompt gamma ray photons and 2 to 3 prompt neutrons are generated in each photofission event, depending on the SNM present. For example, on average, 2.4 neutrons are emitted per fission of uranium-235, while on average 2.9 neutrons are released per fission of plutonium-239. Almost all of the fission fragments are neutron rich and decay toward a stable valley via beta decay to produce delayed gamma rays and delayed neutrons, depending on the fission fragments. These beta decays happen microseconds to hundreds of milliseconds after emission of the prompt gamma rays and neutrons. The emission rate of prompt neutrons and prompt gamma rays is about 100 times greater than the emission rate of delayed neutrons. The emission rate of delayed gamma rays is at least 10 times greater than the emission rate of delayed neutrons. While gamma rays are emitted from almost all of the nuclei subject to fission, not every nucleus will emit a beta delayed neutron. A small amount of fission may be induced by photons having energies below the fission threshold, by barrier penetration. The detection of nuclear material based, at least on part, on the detection of delayed neutrons, is discussed in U.S. Pat. No. 7,423,273, which is assigned to the assignee of the present invention and is incorporated by reference herein.
Neutron detectors, to detect both prompt and delayed neutrons, typically comprise three (3) helium-filled tubes surrounded by a hydrogenous material, such as polyethylene. The hydrogenous material is covered with a layer of cadmium (Cd), which has a large capture cross-section for thermal neutrons (2500 barns). The cadmium absorbs background thermal neutrons resulting from thermalized photoneutron events, preventing their passage to and detection by the helium tubes. Other neutron detection methods are known in the art, including scintillators and silicon carbide (SiC) detectors, for example.
Gamma rays have been collected by scintillator based detectors, such as sodium iodide doped with thallium (NaI(Tl)), barium germanium oxide (BGO), high purity germanium (HPGe), germanium lithium (GeLi), and plastic scintillators. Inorganic scintillators have better efficiency than organic scintillators but may be too expensive for large solid angle arrays required when examining large objects such as cargo containers.